Optically addressed spatial light modulators using liquid crystal layers (hereinafter, referred to as spatial light modulator) basically includes a photoconductive layer, a liquid crystal layer which changes light transmittance upon application of electric field and a pair of transparent conductive electrodes which sandwich the photoconductive layer and the liquid crystal layer between the electrodes.
This modulator is driven by applying voltage to both electrodes from outside. Light irradiated toward the photoconductive layer leads to a change in electrical resistance in the photoconductive layer and a change in the voltage applied to the liquid crystal layer. Accordingly, a read light passing through the liquid crystal layer is modulated according to the variation in voltage. Such operation enables many functions including light threshold processing, wavelength conversion, incoherent/coherent conversion, image memory and the like. Therefore, spatial light modulators are considered an important device for optical processing. Further, these devices serve as optical amplifiers when a read light with a large optical intensity is injected from the opposite direction of light for writing, and the memory is read-out in reflection mode. Accordingly, these devices are used as a general-purpose device including a projection-type display.
Projection processes of conventional projection-type displays include the following three types: (1) one using the above-mentioned optically addressed spatial light modulator, (2) one using bright three-tube cathod ray tube (CRT), and (3) last one which projects an active matrix liquid crystal light valve with a bright light source.
In the process using CRT, images are displayed on three bright CRTs of R, G and B with an opposite angle of 5 to 7 inches, the images are projected on a screen by three projection lenses and synthesized to obtain a color image. This process has problems that the projector is heavy, or a bright display decreases resolution.
In the process using an active matrix liquid crystal light valve, images are displayed on a liquid crystal panel which is formed of three liquid crystal panels or color filters of Red, Green and Blue integrated in one body. The displayed images are read-out by a bright back light source such as a metal halide lamp or a halogen lamp, and are projected on a screen. In contrast to CRT method, this method has the advantages of enabling to downsize the projector. However, the method still has a problem. To obtain a high resolution image, the size of pixels in a liquid crystal panel must be small. The ratio of light-shielding region (a transistor component for driving an active matrix) to the size of the pixels increases with decreasing rate of pixel hole area, thus darkening images. Consequently, as long as projection-type displays using CRT or active matrix liquid crystal light valve were used, the resolution would always be in inverse proportion to the brightness.
On the contrary, when optically addressed spatial light modulators are used, images are input in a photoconductive layer by CRT. The images are read-out from the liquid crystal layer side by a bright light source, and they are projected on a screen through a projection lens. This process can work with a downsized projector to provide a bright image plane with high resolution. Therefore, this new process solves the problems with brightness and resolution, from which conventional displays suffered.
The best conventional spatial light modulator is a device including a thin layer of amorphous silicon hydride (hereinafter, abbreviated as a-Si:H) as a photoconductive layer and a liquid crystal layer formed of ferroelectric liquid crystal (hereinafter, abbreviated as FLC). Such device exhibits the highest sensitivity and the most rapid response, and further is easy to use with a small operation voltage.
FIG. 10 is a sectional view of this device. The device comprises photoconductive layer 1001, reflector 1002, FLC liquid crystal layer 1003, a pair of transparent electrodes 1004 and 1005, and a pair of glass substrates 1006 and 1007 sandwiching those layers, reflector and electrodes therebetween. Photoconductive layer 1001 is a pin diode formed of an a-Si:H thin film. Furthermore, a projection-type display equipped with such a modulator and CRT as an image writing means is disclosed in M. Bone et al., SID Digest, pp. 254-256.
The a-Si:H thin film for a photoconductive layer is formed by plasma chemical vapor deposition (chemical vapor deposition is abbreviated as CVD). Optical conductivities of the a-Si:H thin film strongly depends on the substrate temperature at the film forming step. In general, the substrate temperature at the film forming step is preferably set at approximately 200.degree. to 300.degree. C. Temperatures above 300.degree. C., or below 180.degree. C. are not preferred because the density of dangling bonds in the film inappropriately increases. In particular, the increase in the density of dangling bonds considerably decreases optical conductivity since the dangling bonds trap or recombine carriers. A photoconductive layer in the spatial light modulator produces light-excited carriers upon light irradiation and transports the carriers toward the liquid crystal layer. The photoconductive layer therefore plays an important role in determining sensitivity and response rate of the spatial light modulator. For this reason, the photoconductive layer needs high optical conductivity. To obtain such high conductivity, the a-Si:H films were formed at the substrate temperature of 200.degree. to 300.degree. C. in the plasma CVD or the reactive sputtering.
With projection-type displays using a CRT as a write light source, a CRT with an opposite angle of at least 7 inches was used to write an image in the spatial light modulator. The light emitting wavelength of the CRT was at least 600 nm.
The condition where light is irradiated under reverse bias in conventional photoconductive layer 1001 in FIG. 10 is described below.
Though most of the light irradiated is absorbed in the p/i layer, part of the rest is absorbed in the n layer, or returns to the i layer after passing through the n layer and being reflected back from reflector 1002. No depletion layer extends in the n layer of low-resistance under reverse bias. As a result, electron-hole pairs produced by optical absorption in the n layer do not drift in an electric field. The electron-hole pairs do not work as photocurrent to run in photoconductive layer 1001. Light returned to the i layer from reflector 1002 extends photocarriers horizontally, decreasing resolution of written images. The larger the photocurrent runs in the photoconductive layer with respect to incident light with a constant intensity, the smaller the necessary optical intensity of light for switching the liquid crystal layer is. Sensitivity of the spatial light modulator is thus improved. Consequently, the photoconductive layer for use in conventional spatial light modulators does not utilize write light effectively, decreasing sensitivity of the modulators.
An a-Si:H film for a photoconductive layer is formed by plasma CVD as follows. A substrate in a vacuum chamber is heated up to 200.degree. to 300.degree. C. On the substrate, an a-Si:H film having a given thickness is formed and cooled down to around room temperature. In this process, substrate heating and cooling steps take two hours or more in total. The required time is as long as necessary for forming a film, or more. To shorten the heating time, the heating rate is increased simply by increasing the amount of current to a heater. However, a shortened heating time causes a problem that a modulator cannot output constant images free of irregularity.
On an industrial scale using a great number of substrates at a time, heat is not evenly conveyed to them in a short time, causing unevenness in the substrate temperature and irregularity in sensitivity of the photoconductive layer. To obtain a constant substrate temperature, at least one hour is necessary to heat substrates even if the heating rate is increased. To shorten the cooling time, water cooling seems effective; however such rapid cooling causes film stress, causing peeling or cracking of films. Consequently, substrates after forming films is so far required to be cooled gradually for about one hour. Even if the time for forming an a-Si:H film was shortened, the whole processes to produce a device was dependent on the heating and cooling time. Therefore, devices were not effectively mass-produced.
Conventional projection-type displays using CRT utilize a long-wave light having a wavelength of 600 nm or more to write images. The coefficient of absorptivity of a-Si:H with respect to this wavelength is so small that a photoconductive layer needs a thickness of at least 2 .mu.m in order to absorb the whole light. The photoconductive layer in spatial light modulators using FLC as a liquid crystal layer usually has a thickness of 2 to 3 .mu.m. The thickness has no problem in absorbing write light. However, the electron mobility of a-Si:H is two orders of magnitude larger than the hole mobility, and the trap center of holes is deeper in light energy than that of electrons. Consequently, the response in photocurrent running through the photoconductive layer was late. The displays had a problem that afterimage or burning occurred in the output image. Furthermore, when a CRT which emits light against a short wave light of 600 nm or less is used, the p-type a-Si:H layer has a large light absorption and needs a large amount of light to write images. Nevertheless, output of images with a high brightness using a small CRT considerably decreases resolution and contrast. Therefore, CRT was required to be large for much light, causing enlargement of projectors.